The maximum transmissible optical power in a high-resolution scanning fiber endoscope system is limited by the requirement for small-core singlemode light propagation. The generally accepted threshold for material damage of a singlemode silica optical fiber is about 1 MW/cm2 for solid fiber and higher thresholds for photonics crystal or microstructured singlemode optical fibers. Thus, using an optical fiber that has a mode field diameter of 3.5 microns limits the optical power that can be delivered for therapy to a maximum of about 100 mW, when a conventional single resonant optical fiber is employed for both imaging and therapy. This level of power can easily be provided in the visible wavelengths by currently available diode-pumped solid-state or argon-ion gas lasers, and by high-power ultraviolet laser diodes and ultraviolet lasers that are being developed. Use of light in the visible range of wavelengths is desirable for rendering therapy in current configurations that employ the same optical fiber for both imaging and therapy, because of the potential for increased bending and launching losses that occur if infrared (IR) wavelengths are used in a visible wavelength optical fiber, or if shorter wavelength ultraviolet light is used in conventional multimode optical fiber scanners. Although tissue absorption levels are low in the visible range, it is expected that this amount of power will enable some limited therapeutic capability.
However, in the case where higher levels of optical power are needed for advanced levels of therapy, the existing scanning fiber endoscope design does not provide sufficient power handling capability. This problem only becomes an issue if it is necessary to both image and provide therapy to an internal site. If one or more fixed optical fibers are used to provide therapy, the nature and quantity of the therapeutic fibers and laser sources can be selected solely on the basis of therapeutic effect, with no regard to imaging. For example, the most commonly used laser in digestive endoscopy is the Nd:YAG laser which emits light at 1.06 micron wavelength that is usually conveyed to the tissues by a sheathed optical fiber within the working channel of the endoscope or within a cannula alongside the endoscope (Brunetaud, J. M., Maunoury, V., and Cochelard, D., Lasers in Digestive Endoscopy, Journal of Biomedical Optics 2(1): 42-52 January 1997). To deliver these much greater optical power levels, large-core multimode optical fibers would typically be used, rather than the small-core, singlemode optical fiber that is required for high-resolution imaging. To deliver more than fixed spots of laser irradiation to an imaged field, the separate large-core optical fiber(s) must be inserted through a larger endoscope within a working channel or secondary cannula that allows moving delivery of the optical therapeutic dosage across the stationary endoscopic field by hand. A drawback to this approach is that additional channels are required for combining imaging and therapy for minimally-invasive medicine.
One advantage of employing a dedicated fixed fiber configuration for a separate therapy channel is that it can operate at optical powers below the material damage threshold and still deliver sufficient power to perform a broad range of laser therapies. The disadvantage of such a configuration, however, is that the resulting endoscope system is more bulky and more invasive to the patient. Therefore, it would be desirable to provide a configuration for an endoscope system that can achieve maximal power operation in one single illumination fiber endoscope to provide the desired optical power therapeutic capacity, while also enabling imaging (and perhaps diagnostic) procedures to be conducted of the site to which the optical therapy is to be delivered. A compact single optical fiber endoscope with such properties has not yet been commercially available.